Cache memory device

ABSTRACT

A cache memory device with a cache section, which is provided between a CPU and a main memory and operates as a fast buffer memory, has a capability of storing input data in the cache section when attribute information affixed to the input data indicates a predetermined attribute.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a cache memory device, and, moreparticularly, to a cache memory device which suppresses the occurrenceof replacement of data between a cache and a main memory in dataprocessing such as communication control.

2. Description of the Prior Art

A communication control apparatus is provided to mutually connectcomputer networks constructed by the Ethernet or the like and controlmutual communications among the networks. FIG. 6A is a block diagram ofa router which is one example of the communication control apparatus andFIG. 6B is a diagram showing the structure of a packet. A header 11includes information, such as the IP address of a sender and the MACaddress of a transmission destination, communication data 12 is a databody to be transmitted, and a trailer 13 includes error checkinformation.

A communication control apparatus 61 receives packet data P1 from anetwork to which the apparatus 61 is connected, identifies the IPaddress of a machine/unit which is connected to a network which is wherethe data is to be sent, selects a route to transfer by referring to aroute table, updates the MAC address portion of the header and sends outpacket data PO. The following describes an operation that is carried outhere. In the communication control apparatus 61, a DMA (Direct MemoryAccess) 65 transfers a packet 10 received by a network I/O unit 64,connected to a system bus 62, to a main memory 69, constructed by aDRAM, via a memory interface 68 connected to the system bus 62 andwrites the packet 10 there, after which a CPU 63 refers to the IPaddress of the received packet and updates the MAC address. For fasterprocessing of the CPU 63, the communication control apparatus 61 has acache memory device 67 which constitutes a write-back type secondarycache between the CPU 63 and the main memory 69. Because of thepossibility that data in the main memory 69 may be later than data inthe secondary cache (cache memory device 67), however, data in the cachememory device 67 is temporarily invalidated by using a cache invalidatecommand or a coherency protocol command, and then the cache memorydevice 67 is accessed via a CPU bus 66. As a packet to be accessed isnot located in the cache memory device 67 due to the invalidation of thedata, the cache memory device 67 reads out data from the main memory 69via the memory interface 68. Then, the CPU 63 reads out apart of thepacket 10 including the header 11 from the cache memory device 67,updates the MAC address portion of the header 11 and writes theresultant data in the cache memory device 67. The packet with theupdated header 11 is written back (flushed) into the main memory 69 fromthe cache memory device 67 in response to a command from the CPU 63, isthen transferred via the memory interface 68 and the system bus 62 tothe network I/O unit 64 from the main memory 69 by the DMA 65, and istransferred to another network from the network I/O unit 64.

FIG. 7 is an internal block diagram of the conventional cache memorydevice 67. The cache device 67 has an n-way cache section 72 whichincludes n (a positive integer equal to or greater than 2) ways from afirst way 71-1 to an n-th way 72-n each including m (a positive integerequal to or greater than 2) entries each having a pair of a tag portionfor storing a tag address and a data portion for storing data. That is,the cache section 72 includes n ways each capable of storing m tag anddata pairs. The cache section 72 may be considered as having m sets thatcan designate n entries from the first way 71-1 to the n-th way 71-nwith the same index address. Although not illustrated, a valid bit whichindicates the valid/invalid state of data stored in each entry of thecache section 72 and a dirty bit or the like which indicates a statewhere data on the cache has been updated but has not been written backinto the main memory are provided in association with each entry. Thecache memory device 67 further includes n comparison circuits 73-1 to73-n, an OR circuit 74 and a selection circuit 75. Each of thecomparison circuits 73-1 to 73-n detects a match between a tag addressstored in a tag portion that is specified by an index address ADI in anaddress Ad (see FIG. 4B) having a tag field, an index field and a linefield and an address ADT of the tag field of the address AD. The ORcircuit 74 sets a hit signal HIT active when a match is detected bythose comparison circuits and sends the hit signal HIT to a controlcircuit 77. The selection circuit 75 selects a block of the way(including data of plural lines of line addresses) that corresponds tothe comparison circuit which has detected a match when an access made bythe CPU 63 is a read access. The cache memory device 67 also includes anLRU (Least Recently Used) memory section 76 that stores a block whichbelongs to a set comprising blocks of n ways which have the same indexaddress and which has not been used for the longest period of time foreach set, as a push-out candidate block LRUB. The control circuit 77controls writing and reading of a tag address and data to and from thecache section 72, replacement of data and so forth.

In case where the CPU 63 accesses the cache memory device 67 via the CPUbus 66, when none of the tag addresses of n entries in the set that isspecified by the index address ADI of the address AD from the CPU 63coincides with the address ADT of the tag field of the address AD, i.e.,when a mishit (or “miss”) has occurred, the push-out candidate blockLRUB registered in the LRU memory section 76 is extracted from the setthat is indicated by the index address ADI, the tag address and data inthe associated entry in the cache section 72 are replaced with the tagfield portion and data at the address in the main memory 69 that isindicated by the address AD. Then, data of the block pushed out as thepush-out candidate block LRUB from the cache section 72 is sent to themain memory 69 via the system bus 62 and written in the associated areain the main memory 69 where the address ADT of the tag field correspondsto the index address ADI, all under the control of the control circuit77.

When the CPU 63 makes a write access and detects a hit, dirtyinformation is affixed to write data WD as the associated block in thecache section 72 is rewritten through a switch section 78 controlled bythe control circuit 77. The dirty block data is rewritten in the mainmemory 69 when replacement occurs due to a mishit of an access made bythe CPU 63 or the data in the main memory 69 is updated by executingflushing in response to a command from the CPU 63 before beingtransferred to the network I/O unit 64 from the main memory 69. In caseof a mishit, data MD from the main memory 69 is sent to the cachesection 72 via the switch section 78 controlled by the control circuit77 and written there. Then, the CPU 63 performs overwriting of data inthe cache section 72 and adds dirty information to the associated entryin the cache section 72.

In the conventional communication control apparatus 61 in FIG. 6A, theCPU 63 can access only the cache memory device 67 via the CPU bus 66 andthe network I/O unit 64 can execute DMA with the main memory 69 via thesystem bus 62. The communication control apparatus 61 therefore requiresa process of replacing data of the associated portion into the cachememory device 67 from the main memory 69 for each access made by the CPU63 or flushing data updated by the CPU 63 into the main memory 69 fromthe cache memory device 67 prior to transfer to the network I/O unit 64,during a period from the reception of a packet PI from at the networkI/O unit 64 to the renewal of its header and transmission of theresultant packet as a packet PO from the network I/O unit 64 by the CPU63. The communication control apparatus 61 therefore undesirably takes agreater time in data exchange between the cache memory device 67 and themain memory 69 than processing in the CPU 63.

In addition, a portion of communication data 12 which is not used inupdating communication control information is stored in the cachesection 72 in the conventional cache memory device 67 without beingdiscriminated adequately. This brings about a situation of pushing outdata, such as data of a header portion which is likely to be used anddata which is to be used when a command code and a command from the CPUare executed from the cache section 72 when communication data which isnot used by the CPU is accessed. It is therefore necessary to performdata replacement when the pushed-out header data becomes necessaryagain. This leads to frequent replacement of data blocks between thecache memory device 67 and the main memory 69, thus lowering the hitratio of the secondary cache and lowering the performance of thecommunication control apparatus 61 as a consequence.

BRIEF SUMMARY OF THE INVENTION OBJECT OF THE INVENTION

Accordingly, it is an object of the invention to provide a cache memorydevice that selectively stores data which is likely to be frequentlyused in a cache section.

SUMMARY OF THE INVENTION

A cache memory device with a cache section, which is provided between aCPU and a main memory and operates as a fast buffer memory, has acapability of storing input data in the cache section when attributeinformation affixed to the input data indicates a predeterminedattribute.

BRIEF DESCRIPTION OF THE DRAWINGS

The above-mentioned and other objects, features and advantages of thisinvention will become more apparent by reference to the followingdetailed description of the invention taken in conjunction with theaccompanying drawings, wherein:

FIG. 1A is a block diagram of a communication control apparatus which isone embodiment of a data processing apparatus according to the inventionand FIG. 1B is a diagram showing the structure of an attribute-affixedpacket;

FIG. 2 is an internal block diagram of a cache memory device accordingto a first embodiment of the invention;

FIG. 3 is an internal block diagram of a cache memory device accordingto a second embodiment of the invention;

FIG. 4A is an operational flowchart of a cache memory device 6 b andFIG. 4B is a diagram showing the correlation of tags and indexes in acache section and a non-allocation buffer;

FIGS. 5A and 5B are internal block diagrams of different examples of thenon-allocation buffer;

FIG. 6A is a block diagram of a conventional communication controlapparatus and FIG. 6B is a diagram showing the structure of a packet;and

FIG. 7 is an internal block diagram of a conventional cache memorydevice.

DETAILED DESCRIPTION OF THE INVENTION

The invention will now be described referring to the accompanyingdrawings. FIG. 1A is a block diagram of a communication controlapparatus which is one embodiment of a data processing apparatusaccording to the invention.

A communication control apparatus 1 includes a system bus 2, a CPU 3, anetwork I/O unit 4 and a cache memory device 6, the last three connectedto the system bus 2. The cache memory device 6 is connected via a memoryinterface 7 to a main memory 8 which is constructed by a DRAM. As thecommunication control apparatus 1 performs DMA transfer directly to thecache memory device 6 from the network I/O unit 4, the communicationcontrol apparatus 1 can considerably reduce the number of shifts of datato the cache memory device 6 from the main memory 8 at the time the CPU3 accesses DMA-transferred data, as compared with the conventionalcommunication control apparatus 61 in FIG. 6. In DMA transfer in theopposite direction from the cache memory device 6 to the network I/Ounit 4, data is directly transferred from the cache memory device 6 too.It is therefore possible to eliminate the flush process whichtemporarily writes dirty data back into the main memory and invalidatesan associated entry before transfer and which has been needed in theconventional communication control apparatus 61. Further, a DMA 5incorporated in the network I/O unit 4 has an attribute adding section9. The DMA 5 respectively adds attribute bits 14, 15 and 16 to a header11, a communication data 12 and a trailer 13 as shown in FIG. 1B andexecutes DMA transfer while setting the attribute bit 14 of the header11 which is used at the time of referring to a route table to “1” andsetting the attribute bit 15 of the communication data 12 which is notreferred to by the CPU 3 to “0”. FIG. 1B illustrates a case where theattribute bit 16 of the trailer 13 is also set to “0” and the attributebit 14 of the header 11 alone is set to “1”. Because the cache memorydevice 6 has a capability of discriminating data whose attribute bit is“1” from data whose attribute bit is “0” and allocating data whoseattribute bit is “1” into a cache section, data needed by the CPU 3 isstored in the cache memory device 6. This significantly improves the hitratio and reduces replacement of data between the cache memory device 6and the main memory 8 that is originated from a mishit. In particular,replacement of data which is not accessed by the CPU 3 is suppressed,the process efficiency of communication control is improved.

The operation of the communication control apparatus 1 will now bedescribed. The communication control apparatus 1 receives packet data PIfrom a network to which the apparatus 1 is connected, identifies the IPaddress of a transmission destination of the data, selects a route totransfer by referring to a route table, updates a part of the headeraccording to the selected route and sends out packet data PO. A packetreceived by the network I/O unit 4, connected to the system bus 2, hasthe header 11 affixed with the attribute bit 14 of “1” indicating anallocation attribute to the cache section and the communication data 12and the trailer 13 are respectively affixed with the attribute bits 15and 16 of “0” indicating a non-allocation attribute. The packet is thenDMA-transferred to the cache memory device 6 that is connected to thesystem bus 2 and used as a secondary cache. In the case where a bustransaction is carried out in a fixed size, a transaction which includesboth the data portion of the allocation attribute and the data portionof the non-allocation attribute may be carried out, considering theentire data as having an allocation attribute as a whole. The cachememory device 6 stores the header 11 whose attribute bit is data withthe allocation attribute of “1” in the cache section. In the case wherethe tag address of the transfer destination has a hit, data is writtenin an associated address in the cache section, whereas in the case wherethere is no hit, a block to be replaced is selected by the LRU methodand is written in the main memory 8 after replacement of data and a tagaddress at an associated address in the main memory 8.

The CPU 3 reads out the header 11 from the cache memory device 6, checksrouting information, updates a portion of the header 11 and returns theresultant data to the cache memory device 6. When the transmissiondestination of the packet is determined, the DMA 5 is activated toperform DMA transfer of a packet that has the header 11 allocated in thecache section of the cache memory device 6 and the communication data 12and trailer 13, which have not been allocated in the cache section, tothe network I/O unit 4 via the system bus 2. The packet is thentransferred to another network from the network I/O unit 4.

Although the foregoing description has been given of the case where theheader 11 alone is treated as data with the allocation attribute of “1”,there may be a case where the hit ratio can be further improved if boththe header and the trailer 13 are treated as data with the allocationattribute of “1” and only the communication data 12 is treated as datawith the allocation attribute of “0”.

FIG. 2 is an internal block diagram of a cache memory device accordingto the first embodiment of the invention. A cache memory device 6 a,like the conventional cache memory device in FIG. 7, has an n-way cachesection 22 which includes n ways from a first way 21-1 to an n-th way21-n each including pairs of tag portions and data portions, ncomparison circuits 23-1 to 23-n, an OR circuit 24 and a selectioncircuit 25. Each of the comparison circuits 23-1 to 23-n detects a matchbetween a tag address stored in a tag portion that is specified by anindex address ADI in an address AD, which has a tag field and an indexfield, and an address ADT of the tag field of the address AD. The ORcircuit 24 sets a hit signal HIT active when a match is detected bythose comparison circuits and sends the hit signal HIT to a controlcircuit 27. The selection circuit 25 selects an entry of the way thatcorresponds to the comparison circuit which has detected a match when anaccess made by the CPU 3 is a read access. The cache memory device 6 aalso includes an LRU memory section 26 for storing a block which belongsto a set comprising blocks of n ways that have the same index addressand which has not been used for the longest period of time for each set.

In addition, the cache memory device 6a has a data attribute determiningsection 29 in the control circuit 27. The data attribute determiningsection 29 determines the attribute bit of data transferred from thenetwork I/O unit 4 via the system bus 2 and operates a switch section 28to allocate, by priority, data (header) affixed with “1” indicating anallocation attribute in the cache section 22. That is in the case wherethe tag address of the transfer destination has a hit, data is writtenin an associated block in the cache section 22 and the associated blockis treated as dirty. In the case where there is a mishit, a block to bereplaced by the LRU method is selected. When data of that block isdirty, it is written back into the main memory 8 and data of the blockthat is associated with the address where transfer data from the mainmemory 8 is written is allocated, then the transfer data is overwrittenand affixed with dirty information. With regard to data affixed with anon-allocation attribute data of “0” by the DMA 5, it is first checkedif there is a hit in the cache section 22. When there is a mishit, theswitch section 28 is operated in such a way that the data passes throughthe cache memory device 6 and is output directly as data MD. The data MDis directly written in the main memory 8 via the memory interface 7. Incase where there is a hit in the cache section 22, data in the cachesection 22 is updated to be dirty.

In the case where the CPU 3 accesses the cache memory device 6, the CPU3 informs whether data to be accessed is allocation attribute data ornon-allocation attribute data. For every access from the CPU 3, thecache section 22 is accessed first. When there is a hit and the accesstype is a write access, the control circuit 27 operates the switchsection 28 to rewrite the associated block into the cache section 22 andaffix dirty information to the data. The data of the dirty block iswritten back into the main memory 8 when a mishit-originated replacementtakes place. When the access type is a read access, data of an entry ofthe way whose tag portion has a tag address coincident with the addressADT of the tag field of the address AD is read out as read data DAT.

It is considered as a mishit when none of the tag addresses of n entriesof the set that is specified by the index address ADI of the address ADmatches with the address ADT of the tag field. When the switch section28 determines that data to be accessed is allocation attribute data, ablock LRUB registered as an LRU block in the LRU memory section 26 isextracted from the set that is specified by the index address ADI. Then,the tag address and data of the block of the way that is associated withthe LRUB in the cache section 22 are updated to a tag address and datawhich are associated with the tag field of the address in the mainmemory 8 that is indicated by the address AD, and data selected andpushed out from the cache section 22 by the LRU method is written backinto the memory address in the main memory 8 that corresponds to theindex address, all under the control of the control circuit 27.

When there is a mishit and the switch section 28 determines that data tobe accessed is not allocation attribute data, the main memory 8 isdirectly accessed via the memory interface 7 under the control of thecontrol circuit 27. When the access type is a read access, the addressAD is sent to the main memory 8 and data MD read from the main memory 8is sent through the switch section 28 of the cache memory device 6 aonto the system bus 2. When the access type is a write access, theaddress AD is sent to the main memory 8 and data is sent through theswitch section 28 of the cache memory device 6 a to the main memory 8 tobe written there.

As apparent from the above, because the cache memory device 6 a has acapability of selecting whether to allocate data in the cache section 22or send data to the main memory 8 without allocation depending on theattribute bit added to the data, the cache memory device 6 a canselectively store data which is very likely to be repeatedly accessed inthe cache section 22, thus significantly improving the hit ratio.

FIG. 3 is an internal block diagram of a cache memory device accordingto the second embodiment of the invention. A cache memory device 6 baccording to the embodiment has a non-allocation buffer 31 for storingnon-allocation attribute data so that a part of non-allocation attributedata which is stored in the main memory 8 in the cache memory device 6 ain FIG. 2 is stored in the non-allocation buffer 31. Specifically, byusing a data attribute determining section 33 in the control circuit 27,the cache memory device 6 b determines the attribute bit of datatransferred from the network I/O unit 4 via the system bus 2 andoperates the switch section 28 to allocate data affixed with “1”indicating an allocation attribute to the cache section 22. In the casewhere the tag address of the transfer destination has a hit, data iswritten in an associated block in the cache section 22 and is affixedwith dirty information. In the case where there is a mishit, a block tobe replaced by the LRU method is selected and is written back into themain memory 8, data of the block that is associated with the addresswhere transfer data from the main memory 8 is written is allocated, thenthe transfer data is overwritten and affixed with dirty information.With regard to data affixed with non-allocation attribute data of “0”,the switch section 28 is operated in such a way that the data istransferred to the non-allocation buffer 31. When the tag address of thetransfer destination has a hit, the data is written in the associatedblock in the non-allocation buffer 31 and affixed with dirtyinformation. When there is a mishit, an entry indicated by an indexaddress ADIb is selected and written back into the main memory 8, dataof the block that is associated with the address where transfer datafrom the main memory 8 is written at the associated address in thenon-allocation buffer 31, then the transfer data is overwritten andaffixed with dirty information.

Like each way of the cache section, the non-allocation buffer 31 has atag portion and a data portion. It is to be noted however that thecapacity of the non-allocation buffer 31 can be smaller than thecapacity of a single way of the cache section 22, not to mention thatthe capacity of the non-allocation buffer 31 is smaller than the entirememory capacity of the cache section 22. In this case, as shown in FIG.4B, a field which is assigned to the index address ADIb in thenon-allocation buffer 31 becomes smaller than the field of an indexaddress ADIa in the cache section 22, and the field of a tag addressADTb which is assigned to the non-allocation buffer 31 becomes greaterthan the field of a tag address ADTa in the cache section 22accordingly. Therefore, the number of bits of the tag address to bestored in the tag portion of the non-allocation buffer 31 is greaterthan the number of bits of the tag address to be stored in the tagportion of each way in the cache section 22.

FIG. 4A is an operational flowchart of the cache memory device 6 b. Whenthe CPU 3 makes an access, in first step 41, tag addresses stored in thetag portions of n ways in the cache section 22 that are indicated by thefirst index address ADIa of the address AD to be accessed are comparedwith the address ADTa of the tag field of the address AD, tag addressesstored in the tag portions that are indicated by the index address ADIbin the non-allocation buffer 31 are simultaneously compared with theaddress ADTb of the address AD for the non-allocation buffer, and the ORcircuit 24 sets the hit signal HIT active and outputs the signal HITwhen any one of the comparison circuits 23-1 to 23-n corresponding tothe n ways of the cache section 22 and a comparison circuit 32corresponding to the non-allocation buffer 31 detects a match. In nextstep 42, it is determined whether or not there is a hit. When it isdetermined that there is a hit, the flow goes to step 46, whereas whenit is determined that there is no hit, the flow goes to step 43.

In step 43, it is determined whether or not the type of data that hashad a mishit is allocation attribute data. This decision is made by thedata attribute determining section 33 based on the attribute of the datasent from the CPU 3 at the time an access is made. When it is determinedthat data to be accessed is allocation attribute data, the flow proceedsto step 44 to select a block in the cache section 22 which is to bereplaced by the LRU method and replace the block with data and the tagaddress of the associated address in the main memory 8. The data MD fromthe main memory 8 is sent to the cache section 22 through the switchsection 28 under the control of the control circuit 27. Data of theblock that is to be pushed out from the cache section 22 is written backinto the main memory 8. When it is determined in step 43 that data to beaccessed is non-allocation attribute data, the flow proceeds to step 45to replace the entry which is indicated by the index address ADIb in thenon-allocation buffer 31 with data and the tag address of the associatedaddress in the main memory 8. The data MD from the main memory 8 is sentto the non-allocation buffer 31 through the switch section 28 under thecontrol of the control circuit 27. Data of the block that is to bepushed out from the non-allocation buffer 31 is written back into themain memory 8.

After step 44 or step 45 is finished, the flow returns to step 41. Innext step 42, there is certainly a hit, so that the flow proceeds tostep 46 to determine whether or not the access type is a read access.When it is not determined that the access type is a read access, theflow goes to step 47. As the access type is a write access in this case,write data WD is sent through the switch section 28 to the way in thecache section 22 that has had a hit or the non-allocation buffer 31 tobe rewritten there and affixed with dirty information, after which theaccess process is terminated. When it is determined as a read access instep 46, data of a block in the way in the cache section 22 that has hada hit or the non-allocation buffer 31 is sent as read data DAT to theCPU 3 via the system bus 2, after which accessing by the CPU 3 isterminated.

The cache memory device 6 b can store a part of data which is notaffixed with an allocation attribute in the non-allocation buffer 31.Under the communication circumstance where there is a locality on time,such as a very short time involved from the transfer of a packet to thecache memory device 6 b from the network I/O unit 4 to the transmissionof the packet to the network I/O unit 4 from the cache memory device 6 bafter being processed by the CPU 3 or under the communicationcircumstance where there is a spatial locality, such as the same packetbeing frequently exchanged between a sender and a receiver, theprobability of accessing communication data and a trailer while they arestored in the non-allocation buffer 31 is increased, so that thenon-allocation buffer 31 effectively serves as a cache memory fornon-allocation attribute data. This can also shorten the access time fornon-allocation attribute data. In the case where the non-allocationbuffer 31 is designed to have the same structure as that of a single wayof the cache section 22, memory blocks of the ways in the cache section22 can be used directly as if the non-allocation buffer 31 served as a(n+1)-th way in the cache section 22. In addition, the index field andtag field of the address AD can be made common and the comparisoncircuits 23-1 to 23-n can be used for the comparison circuit 32. Thisbrings about an advantage of reducing the number of design steps.

FIGS. 5A and 5B are internal block diagrams of different examples of thenon-allocation buffer. A non-allocation buffer 51 in FIG. 5A has amemory area where four pairs of tags and data can be entered and a timeris provided for each tag and data pair. The non-allocation buffer 51 inthis example restricts the time for non-allocation attribute data tostay in the non-allocation buffer. In case where the CPU 3 makes anaccess to an address whose index address corresponds to a pair of a tagB and data B and the tag address does not have a match, resulting in amishit and replacement of data between the non-allocation buffer 51 andthe main memory 8, the timer is activated in the non-allocation buffer51 at the point of time when data replacement has taken place. When apreset time is measured, the control circuit 27 is informed of atime-out TOB and non-allocation attribute data is written back into themain memory 8. As data whose access probability can be considered ashaving become smaller is written back into the main memory beforehand, aprocess of writing data stored so far in the non-allocation buffer 51back into the main memory 8 can be eliminated at the time oftransferring a packet from the network I/O unit 4. This prevents theeffectiveness of the provision of the non-allocation buffer from beingdeteriorated under the communication circumstance where there is a timelocality or spatial locality. The non-allocation buffer 51 may bedesigned in such a way that the timer is reset and restarts every timethere is a hit and the buffer 51 is accessed, and the control circuit 27is notified of the time-out TOB and non-allocation attribute data iswritten when the preset time is measured.

A non-allocation buffer 52 in FIG. 5B has a memory area where four pairsof tags and data can be entered and a counter is provided for each tagand data pair. In the non-allocation buffer 52 of this example, whennon-allocation attribute data has been accessed a preset number oftimes, it is determined that further accessing to the non-allocationattribute data is less likely to occur, the non-allocation attributedata is written back into the main memory 8. Predetermined values areset in counters A to D beforehand, the counter value of the counter B iscounted down by “1” every time an access made by the CPU 3 has a hitwith respect to a pair of the tag B and data B, the control circuit 27is notified of a count-out COB and the data is written back into themain memory 8 when the counter value reaches “0”. As per the example inFIG. 5A, at the time of transferring a packet from the network I/O unit,it is possible to eliminate a process of writing data stored so far inthe non-allocation buffer 52 back into the main memory 8. Because data,such as a video stream, has a characteristic that the data, once writtenand read out, will not be accessed again, the feature is effective underthe circumstance where such packet data is frequently processed.

Although the foregoing description in conjunction with FIGS. 5A and 5Bhas been given of the case where there are four pairs of tags and data,the number of pairs is in no way limited to four.

Because the cache memory device according to the invention selectivelystores data which is considered as likely to be frequently used in thecache section, the hit ratio of the cache memory device is improved,thus reducing the number of shifts/replacements of data.

Further, the data processing apparatus according to the inventionseparates data input to the I/O unit to data which is determined asbeing likely to be used and data which is determined as being lesslikely to be used and directly exchanges the data with the cache memorydevice of the invention. This can further reduce the number ofshifts/replacements of data between the cache memory device and the mainmemory and thus demonstrates a significant effect of improving theperformance of the communication control processing.

Although the invention has been described with reference to specificembodiments, this description is not meant to be construed in a limitingsense. Various modifications of the disclosed embodiments will becomeapparent to persons skilled in the art upon reference to the descriptionof the invention. It is therefore contemplated that the appended claimswill cover any modifications or embodiments as fall within the truescope of the invention.

1. A cache memory device, provided between a CPU and a main memory, foroperating as a fast buffer memory, said cache memory device comprising:a cache section for storing a tag and data as a pair; and a controlcircuit having a data attribute determining function, wherein input datais stored in said cache section when said control circuit determinesthat attribute information affixed to said input data by a Direct MemoryAccess (DMA) indicates a predetermined attribute, said input data istransferred to said main memory when said control circuit determinesthat said attribute information of said input data does not indicatesaid predetermined attribute, and when there is a reading access or anexternal writing access, said cache section is accessed first, and, in acase where a mishit occurs and said control circuit determines thatattribute information of data to be accessed indicates saidpredetermined attribute, a block to be replaced is selected from saidcache section by a predetermined method and a tag and data of said blockto be replaced are replaced with a tag of an access address where saidreading or writing access has been made and data is stored at an accessaddress of said main memory, then a process for said reading or writingaccess is executed, and, in a case where a mishit occurs and saidcontrol circuit determines that said attribute information of said datato be accessed does not indicate said predetermined attribute,permitting transmission and reception of those address and data whichare associated with said reading or writing access to and from said mainmemory without intervention of said cache section.
 2. The cache memorydevice according to claim 1, wherein said cache section has n ways intoeach of which entries of a predetermined number of tag and data pairscan be made, where n is a positive integer equal to or greater than 2.3. A cache memory device, provided between a CPU and a main memory, foroperating as a fast buffer memory, said cache memory device comprising:a cache section for storing a tag and data as a pair; a non-allocationbuffer which has a smaller capacity than that of said cache section andstores a tag and data as a pair; and a control circuit having a dataattribute determining function, wherein input data from a Direct MemoryAccess (DMA) is stored in said cache section when said control circuitdetermines that attribute information affixed to said input dataindicates a predetermined attribute, said input data is stored in saidnon-allocation buffer when said control circuit determines that saidattribute information of said input data does not indicate saidpredetermined attribute, and when there is a reading access or anexternal writing access, said cache section and said non-allocationbuffer are simultaneously accessed first, in a case where a mishitoccurs and said control circuit determines that attribute information ofdata to be accessed indicates said predetermined attribute, a block tobe replaced is selected from said cache section by a predeterminedmethod and a tag and data of said block to be replaced are replaced witha tag of an access address where said reading or writing access has beenmade and data is stored at an access address of said main memory, then aprocess for said reading or writing access is executed, and in a casewhere a mishit occurs and said control circuit determines that saidattribute information of said data to be accessed does not indicate saidpredetermined attribute, a block indicated by an index address of saidnon-allocation buffer is treated as a block to be replaced, said tag anddata of said block to be replaced are replaced with said tag of saidreading or writing access address and data stored at said access addressof said main memory, then said process for said reading or writingaccess is executed.
 4. The cache memory device according to claim 3,wherein said non-allocation buffer is additionally provided with a timerin association with each of a pair of said tag and data, starts saidtimer at a time data replacement occurs between said non-allocationbuffer and said main memory, and writes data stored by said replacementback into said main memory when said timer measures a preset time. 5.The cache memory device according to claim 3, wherein saidnon-allocation buffer is additionally provided with a timer inassociation with each of a pair of said tag and data, starts said timerevery time there is a hit and said non-allocation buffer is accessed,and writes data at an associated address back into said main memory whensaid timer measures a preset time.
 6. The cache memory device accordingto claim 3, wherein said non-allocation buffer is additionally providedwith a counter in association with each of a pair of said tag and datafor counting a number of accesses, and writes data at that address whosenumber of accesses has reached a preset access number into said mainmemory.